1. Field of the Invention
The present invention relates to a fault determining apparatus, a fault determining method and an engine control unit for a variable valve timing mechanism for changing an opening/closing timing of an intake valve and/or an exhaust valve by changing a cam phase of an intake cam and/or an exhaust cam with respect to a crank shaft of an internal combustion engine.
2. Description of the Prior Art
A variable valve timing mechanism is provided for improving the exhaust gas characteristic and power of an internal combustion engine by changing an opening/closing timing of intake/exhaust valves in accordance with an operating condition of the internal combustion engine to change valve overlapping and the like. A fault determining apparatus is conventionally known as well for determining a fault in the operating condition of the variable valve timing mechanism, for example, as disclosed in Laid-open Japanese Patent Application No. 8-232617. This variable valve timing mechanism is hydraulically driven by a working oil to change a relative angle of a cam shaft to a cam pulley, thereby changing a cam phase. The cam phase is controlled in a feedback manner to reach a target cam phase which has been set in accordance with an operating condition of an engine.
The aforementioned fault determining apparatus determines that the variable valve timing mechanism is faulty when an absolute value of a difference between the target cam phase and a detected actual cam phase (actual cam phase) is equal to or more than a first predetermined value, and an absolute value of a variation in the actual cam phase is equal to or less than a second predetermined value, because there is a large difference between both cam phases, and the actual cam phase does not properly follow the target cam phase in spite of the fact that the actual cam phase is not rapidly changing toward the target cam phase. The fault determining apparatus also detects the temperature of the working oil and disables a fault determination or extends a determination time when the detected temperature is not within a predetermined range, thereby preventing an erroneous determination when the variable valve timing mechanism exhibits a low responsibility due to a cold working oil and a resulting large viscosity thereof.
As described above, the conventional fault determining apparatus is configured to disable a fault determination or extend a fault determination time when the working oil is at low temperatures. For this reason, when the fault determination is disabled due to a low oil temperature, the fault determining apparatus cannot make the fault determination until the oil temperature rises to the predetermined range, thereby failing to provide a determination result. Also, when the fault determination time is extended, the fault determining apparatus takes a long time for the determination, failing to shortly provide a determination result. Particularly, since a recent tendency is such that the use of the variable valve timing mechanism is increasingly required in an oil temperature range extending to a low oil temperature region for improving the exhaust gas characteristic and product features, an appropriate fault determination becomes correspondingly important in a low oil temperature range.